As an image sensor of an imaging apparatus which acquires image data, an area image sensor device which is a photoelectric converting element which utilizes a semiconductor such as a CMOS (Complementary Metal Oxide Semiconductor) is widely utilized.
One of performance characteristics of the image sensor is a dynamic range. The dynamic range refers to a range of brightness of incident light which can be converted as an effective image signal. A large dynamic range means that both dark light and bright light can be converted into image signals, and means that an image sensor has good performance. Although multiple devices to expand a dynamic range of an image sensor have been proposed, among these devices, there is a method of performing exposure control which is different per pixel.
A photodiode (PD) is provided to each pixel of a normal image sensor, and light incident on the photodiode associated with each pixel is photoelectrically converted into a charge. The PD of each pixel has a fixed charge amount which can be accumulated and therefore causes an overflow of a charge when receiving strong incident light and is placed in a saturated state in which no more signals can be accumulated, and, as a result, a signal equal to or more than a saturation level cannot be extracted. Further, noise generated by pixels and a reading circuit cancels a charge produced by too weak incident light, and therefore signals cannot be extracted.
To obtain an effective image signal, it is necessary to adjust exposure such that light of an adequate intensity is incident on each pixel of an image sensor. By making a mechanism of adjusting this exposure per pixel according to a light intensity per scene, it is possible to expand a dynamic range of an image sensor.
The technique based on this idea is, for example, a technique disclosed in Patent Document 1 (Japanese Translation of PCT Application Laid-Open No. 2003-527775). This technique employs a configuration of dividing an exposure period into a plurality of sub periods, and distributing a binary signal as to whether or not to integrate an exposure signal of each sub period, to each pixel through program (Prg) signal lines in a row direction and a column direction. According to this configuration, exposure is controlled per pixel.
Further, Non-Patent Document 1 (HAMAMOTO, Takayuki and AIZAWA, Kiyoharu, “Design and Implementation of Adaptive-integration-time Image Sensor”, Journal of the Institute of Image Information and Television Engineers: Image Information Media, Vol. 55(2), page 271 to 278, February, 2001) discloses a configuration of controlling exposure per pixel by inspecting an accumulated charge which is being exposed per pixel assuming that each pixel is in a non-destroyed state and immediately resetting the charge of a pixel which is decided to be saturated.
Furthermore, Patent Document 2 (Japanese Patent Application Laid-Open No. 2010-136205) discloses a configuration of controlling exposure per pixel by means of a means which determines one of two types of a long exposure time or a short exposure time per pixel, based on an output of an image sensor and a pixel circuit which can control timings to reset and transfer a charge per pixel.